As it is known, block tools have already been proposed, usually including thermosetting resins or sintered metal powders and designed to be mounted on suitable heads or assemblies of a machine tool for carrying out finishing operations by abrasion, e.g. smoothing or polishing surfaces of stony materials (natural stones, such as marbles, granites, etc.), agglomerates, such as glazed gres, resin-quartz, resin-granite, resin-marble, resin-cement, cement-quartz, cement-granite, cement-marble agglomerates, and to be subjected, in use, to a rotating or oscillating movements, or also rotating and oscillating movements.
The block tools used at present for finishing operations of the above mentioned type have, however, various drawbacks, e.g. brittleness, relatively high tendency to undergo abrasion, as well as high costs.
Conventional block tools are obtained according to various manufacturing methods. According to a first method use is made of a press with mold/s and counter-mold/s that have to be heated to a temperature greater than that at which a specific thermosetting resin can be molded. Moreover, it is difficult to obtain uniform heat propagation towards the tool core, which results in inner and outer portions of the block being kept for different “holding” times at the ideal molding temperature, and thus the resulting tool could have portions with different mechanical characteristic features. Moreover, with a heated mold and counter-mold method, it is highly risky to mold a block tool provided with a separate coupling member for attachment to the machine as such a member would prevent, or otherwise hinder, heat propagation within the mold and counter-mold assembly.
A second method comprises casting semi-liquid or pasty inert fillers and thermosetting resins. However, such a method faces problems in relation to sedimentation and separation of the various components as liquids and resins have different specific weights, and obviously heavier components settle, which results in a great unevenness in the composition of the block tool. Moreover, when using such a method an exothermic chemical reaction between polyester or epoxidic resins and the used catalysts (e.g. peroxides, amines) can be produced, which is, inter alia, difficulty to control as it is highly dependant upon ambient temperature that often causes cracks or deformations in the block tool. Finally, with this method the resin percentage must be sufficient to guarantee that a sufficiently fluid mixture is obtained to make it possible to carry out casting without defects, and this requirement is a limitation to the mechanical characteristics of the tools thus obtained.
Moreover, the thermosetting resins that can be used usually need a post-polymerisation heat treatment (postcuring), usually from about 100° to 150° C., thereby increasing the distortion temperature TG, and consequently the mechanical characteristics, up to acceptable values. Preferably, after postcuring treatment, the distortion temperature is lowered to about 60°-120° C.
Another method of producing block tools comprises the use of granular abrasive material (typically industrial diamonds) bonded together by a “metal sintered” binder. In such a method metal powders are used having granular size of the order of some microns, which are mixed in various proportions, and then loaded into a press or sintering machine, and finally subjected to a sintering process.
Particles having size of the order of microns risk to become oxidized by air, which would be detrimental to the efficiency of the sintering operation. Accordingly, such a method of producing block tools requires permanent monitoring and correct powders preservation (usually in a container provided with dehumidifying resin cartridges or under vacuum). This method thus involves high costs for being carried out and for raw materials, as well as problems of metal pollution in waste working water, besides any problems connected to heat shock of the industrial diamond during sinterization with the serious risk of obtaining a permanently weakened structure.